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Can you post a sound clip?
Thoughts on springs the centrifugal force spinning in neutral shouldn't make any noise, only when the clutch is in and the transmission it not spinning. Do you have any free play in the pedal where the throw out bearing isn't touching?
I had the same problem some time ago and my spring on my clutch fork was bad and it was loose, causing the bearing to rattle while the engine was running. When I pressed the clutch peddle in, it would stop. Replaced the spring and adjusted the clutch and everything was OK.